WEBVTT
Kind: captions
Language: en-US
00:00:02.840 --> 00:00:06.720
Let’s try again to get an idea of what these numbers might be
00:00:06.720 --> 00:00:08.840
numerically, what’s this threshold numerically?
00:00:08.880 --> 00:00:10.940
Well, it depends very heavily on the code.
00:00:10.940 --> 00:00:15.820
It depends on the structure of the code, how many qubits you use to encode something.
00:00:15.920 --> 00:00:19.360
What are the operations you need to do the parity checks for,
00:00:19.360 --> 00:00:20.940
how many gates do I need
00:00:21.100 --> 00:00:23.600
to extract this parity information and error correction
00:00:23.820 --> 00:00:26.460
and importantly, when it comes to hardware design,
00:00:26.680 --> 00:00:30.060
what is the architectural structure of the code.
00:00:30.060 --> 00:00:31.720
Now, what do I mean by that?
00:00:31.840 --> 00:00:35.100
If I build an architecture that, what we call, is linear,
00:00:35.100 --> 00:00:37.660
so all my qubits can talk to their neighbors to their left
00:00:37.660 --> 00:00:41.200
and to their right, what we would call a linear geometry.
00:00:41.200 --> 00:00:45.680
You can have a two-dimensional nearest-neighbor geometry where qubits can talk left and right
00:00:45.720 --> 00:00:47.220
and to the ones up and down.
00:00:47.400 --> 00:00:50.540
Or you can have a geometry, which is sort of all connected.
00:00:50.540 --> 00:00:54.280
Any qubit can talk to any other qubit, no matter where they are in the computer.
00:00:54.280 --> 00:00:56.500
You don’t see too many of those architectures.
00:00:56.640 --> 00:00:58.440
Getting back to your earlier question,
00:00:58.440 --> 00:01:00.600
the actual numerical thresholds,
00:01:00.960 --> 00:01:06.260
the best ones that we see for these architectures, which are based on these surface codes or topological codes,
00:01:06.460 --> 00:01:08.760
have a threshold of about 1%.
00:01:09.300 --> 00:01:12.080
Our error rates on each individual qubit,
00:01:12.140 --> 00:01:16.420
each individual gate in our quantum computer has to be below about 1%.
00:01:16.700 --> 00:01:19.360
If we can achieve that experimentally,
00:01:19.360 --> 00:01:22.780
we can think about that this system is scalable
00:01:22.780 --> 00:01:24.800
in the sense that in principle
00:01:24.920 --> 00:01:27.880
we could build an error corrected system that’s very, very large
00:01:28.080 --> 00:01:31.820
and therefore have a quantum computer that contains many, many well-protected qubits.
00:01:32.320 --> 00:01:35.580
So, 1% error rate would allow us to run
00:01:36.180 --> 00:01:38.360
error correction and fault tolerance successfully.
00:01:38.540 --> 00:01:40.080
It would be a good start;
00:01:40.220 --> 00:01:44.760
1% is sort of the – you hit threshold at 1%.
00:01:45.860 --> 00:01:48.260
Hitting threshold doesn’t actually help you very much
00:01:48.260 --> 00:01:51.000
because if you are sitting right at that 1%,
00:01:51.000 --> 00:01:54.340
you need an infinity number of qubits to do anything.
00:01:54.340 --> 00:01:57.860
You just have to sit a little bit below it, so our targets at the moment
00:01:57.860 --> 00:01:59.460
are about 0.1%
00:01:59.740 --> 00:02:03.300
of errors for the actual hardware systems that are being developed.
00:02:03.680 --> 00:02:06.020
Where do we sit today with
00:02:06.020 --> 00:02:09.940
actual hardware development, so the quantum computers that are in the laboratory today,
00:02:09.940 --> 00:02:11.340
how close are they to that?
00:02:11.440 --> 00:02:13.360
Closer than you would think.
00:02:14.300 --> 00:02:18.000
Many people may have seen in the news reports, especially with larger companies,
00:02:18.000 --> 00:02:19.320
such as Google and IBM,
00:02:19.320 --> 00:02:22.660
running what we call superconducting qubit designs.
00:02:23.480 --> 00:02:27.600
They have demonstrated all these QVC and error correction protocols,
00:02:27.760 --> 00:02:31.400
either they are using the surface code or using the original code of Shor.
00:02:32.160 --> 00:02:34.480
Some of them have done multiple versions of this.
00:02:36.080 --> 00:02:38.280
The group at Google has released
00:02:38.460 --> 00:02:41.680
data that suggested their error rates are close
00:02:41.820 --> 00:02:46.040
to what is required. Not quite at that 0.1% level, but they are getting close.
00:02:46.920 --> 00:02:50.760
Systems such as ion traps again have been showing error rates
00:02:50.880 --> 00:02:52.620
that are pretty close to what’s needed,
00:02:53.000 --> 00:02:54.400
but not quite there yet.
00:02:54.400 --> 00:02:58.580
Then, there are several other systems that are not as well developed,
00:02:59.140 --> 00:03:03.300
but certainly have some very good properties that would allow them to scale to the millions,
00:03:03.440 --> 00:03:06.060
if not billions, of qubits that we are going to need
00:03:06.760 --> 00:03:09.500
in order to run interesting quantum algorithms.
00:03:11.480 --> 00:03:13.060
We are recording this in
00:03:13.620 --> 00:03:16.580
early 2018 and you’re saying we’re actually
00:03:16.860 --> 00:03:20.340
close to this threshold value in terms of error correction
00:03:20.460 --> 00:03:24.100
or error rates in physical systems that we can build today.
00:03:24.100 --> 00:03:25.460
Yes, I would expect
00:03:25.460 --> 00:03:30.120
in the not too distant future, within a year or so, multiple systems will hit
00:03:30.300 --> 00:03:31.240
threshold
00:03:31.400 --> 00:03:35.420
on this surface code at least, this 1% error rate that we require.
00:03:35.420 --> 00:03:39.260
We will see this happening in multiple systems. I would say within the next 12 months.
00:03:39.480 --> 00:03:40.480
Excellent!
00:03:42.000 --> 00:03:45.580
With the threshold being about 1%
00:03:45.580 --> 00:03:47.220
and you say we’re targeting
00:03:47.580 --> 00:03:50.740
actual physical error rates of maybe 0.1%
00:03:51.060 --> 00:03:52.480
is what we would like to have.
00:03:53.780 --> 00:03:56.460
If we get to that value, how big is the system going to be,
00:03:56.460 --> 00:03:59.600
how many physical qubits are we going to have to have inside one of these systems?
00:04:00.660 --> 00:04:02.320
It depends what you want to do with it.
00:04:02.320 --> 00:04:06.720
At the moment, a lot of people are trying really, really hard to find interesting
00:04:06.720 --> 00:04:09.180
and commercial quantum algorithms
00:04:10.300 --> 00:04:14.420
that don’t require too many qubits and therefore doesn’t require a lot of error correction.
00:04:14.420 --> 00:04:15.940
They haven’t been found yet.
00:04:16.900 --> 00:04:18.740
I don’t want to sound too pessimistic,
00:04:19.020 --> 00:04:21.940
but to run something of, let’s say, commercial relevance,
00:04:21.940 --> 00:04:25.260
something somebody would pay for either scientifically or commercially;
00:04:25.420 --> 00:04:30.080
you really need to start talking about 10 million qubits in your entire computer.
00:04:30.200 --> 00:04:32.500
You need to be talking at that scale.
00:04:32.840 --> 00:04:35.700
Because again, you also don’t just want one quantum computer
00:04:36.300 --> 00:04:37.260
and then we stop.
00:04:38.160 --> 00:04:39.880
Everybody is going to want a quantum computer,
00:04:39.880 --> 00:04:45.580
so we need these production facilities to be able to build a lot of these things very, very fast
00:04:45.580 --> 00:04:48.120
and most importantly very, very cheaply.
00:04:48.660 --> 00:04:50.720
Qubits, at the moment, are expensive
00:04:51.160 --> 00:04:55.820
and if you need a 100 million qubits or 200 million qubits to do a calculation,
00:04:56.720 --> 00:04:58.740
if they’re costing a $1000 each,
00:04:58.900 --> 00:05:00.600
that computer’s going to be
00:05:00.820 --> 00:05:02.960
a bit too expensive for most people.
00:05:04.300 --> 00:05:08.980
My Ph.D. student, Shota Nagayama, who graduated in 2017,
00:05:09.680 --> 00:05:12.780
his Ph.D. thesis is actually about architectures
00:05:13.300 --> 00:05:15.200
that will allow
00:05:15.720 --> 00:05:20.000
the system to continue to work even if not all of the qubits actually work physically.
00:05:20.120 --> 00:05:24.040
You were a collaborator on that actually. We were very happy to have you involved in that.
00:05:24.040 --> 00:05:26.040
Thank you. I was happy to be involved in that one.
00:05:26.720 --> 00:05:27.520
What do you think that
00:05:27.660 --> 00:05:30.000
does for our ability to actually
00:05:30.120 --> 00:05:33.360
manufacture large scale systems, what kind of impact is that going to have?
00:05:33.400 --> 00:05:35.920
As you know through the work that we did together,
00:05:36.020 --> 00:05:40.100
we tried to design these systems such that they have a certain level of
00:05:40.220 --> 00:05:43.300
– we can’t use the word ‘fault tolerance’ in this context because we have already used it,
00:05:43.300 --> 00:05:45.460
so we will call it ‘defect tolerance’
00:05:45.700 --> 00:05:48.880
where as we manufacture qubits, not all of them work,
00:05:48.900 --> 00:05:51.400
not all of them work well enough to put into our machine.
00:05:51.700 --> 00:05:55.380
The designs, that you and I have worked on and other people have worked on, are there
00:05:56.340 --> 00:05:57.960
to combat this problem,
00:05:57.960 --> 00:06:03.180
but realistically your manufacturing process, you want to be as good as possible.
00:06:03.740 --> 00:06:04.820
I want to go back to
00:06:05.680 --> 00:06:07.100
the issue of how many
00:06:07.100 --> 00:06:09.800
qubits and physical devices we actually have in the system.
00:06:09.800 --> 00:06:10.960
What we had
00:06:10.960 --> 00:06:13.920
kind of motto [ph] on here and we’ve talked about the architectures,
00:06:14.280 --> 00:06:16.260
one of the architectures we talked about is the
00:06:16.400 --> 00:06:19.800
photonic architecture, which I think you were involved in that work.
00:06:20.800 --> 00:06:22.980
Kae talked about the distinction between
00:06:23.280 --> 00:06:27.400
the photons, which carry the qubits and the physical devices that make up the system.
00:06:28.420 --> 00:06:33.380
Yeah. You end up using the devices just to produce more and more photons
00:06:33.380 --> 00:06:34.940
in order to run your computer.
00:06:34.960 --> 00:06:39.880
Linear optics technology is another version of this where the qubits themselves
00:06:40.440 --> 00:06:44.220
– you’re not technically creating them with the devices
00:06:44.220 --> 00:06:47.380
or maintaining them with the devices that you would actually build.
00:06:47.500 --> 00:06:50.380
The devices are sort of there to mediate
00:06:50.720 --> 00:06:52.860
quantum gate operations instead.
00:06:52.860 --> 00:06:55.120
Those are the things you really have to count.
00:06:55.420 --> 00:06:59.900
In this kind of system, we’re counting the number of physical devices as opposed to the number of qubits.
00:06:59.900 --> 00:07:04.160
Exactly! We are counting the number of things we have to build, the number of things we have to pay for.
00:07:04.280 --> 00:07:07.280
That doesn’t, unfortunately, make the numbers any better.
00:07:08.160 --> 00:07:11.740
Even if my quantum computer ? if it were, say, a superconducting quantum computer,
00:07:11.740 --> 00:07:14.880
if that required 10 million qubits to do something,
00:07:14.880 --> 00:07:19.480
even within the atom-optics design that I worked on with Kae, we still need to manufacture
00:07:19.680 --> 00:07:25.220
10 million of these devices that actually form the foundation of the photonics computer.
00:07:25.700 --> 00:07:27.440
There are no free lunches in this game.
00:07:28.260 --> 00:07:32.000
You need a lot of qubits and, unfortunately, it doesn’t matter which system you’re talking about.
00:07:32.500 --> 00:07:34.100
You will always need a lot of qubits.
00:07:35.460 --> 00:07:39.120
Simon, thanks for being with us here and sharing your expertise with us on
00:07:39.120 --> 00:07:43.520
Quantum Error Correction and good luck on starting up your own company with Turing.
00:07:43.580 --> 00:07:46.520
Wonderful! It has been an absolute pleasure, Rod. Thanks for having me.